Our global aim is to develop a robust pulmonary gas exchange imaging method using hyperpolarized 129Xe MRI, which can be used clinically and in small animal research of pulmonary diseases involving gas exchange abnormalities. We propose to significantly improve the resolution and accuracy of our recently demonstrated 129Xe gas exchange imaging method, test its detection limits, compare the method to its most likely competition - high-resolution CT, and compare it to the gold standard - histology. A noninvasive and effective biomarker could lead to a dramatic change in our ability to diagnose, treat, and understand interstitial lung diseases. This research will be carried out in rats and lays the groundwork for anticipated clinical translation. Our method uses hyperpolarized 129Xe to differentially image 129Xe uptake into pulmonary blood/gas barrier and red blood cells. Hyperpolarization increases the 129Xe MRI signal by 5 orders of magnitude compared to thermal polarization, making high resolution gas imaging possible. The large 129Xe chemical shift enables us to discriminate 129Xe in airspace, blood/gas barrier, and red blood cell compartments. For 129Xe to reach the red blood cells, it must first traverse the blood/gas barrier. Since diffusive transport times scale as the square of barrier thickness, imaging of 129Xe uptake in red blood cells is exquisitely sensitive and capable of detecting 5[unreadable]m changes in barrier thickness. Until now, direct imaging of the gas exchange process has not been possible by any method. In addition to ventilation and perfusion, gas exchange is the most fundamental aspect of lung function. Impairment of pulmonary gas exchange occurs in numerous pathologies, but is difficult to assess definitively at early stages when intervention is most likely to be effective. Diseases such as pulmonary fibrosis, inflammation, and radiation induced pneumonitis initially may not dramatically change ventilation or perfusion. Only once the disease is very advanced, can significant structural abnormalities be diagnosed via density increases noted on high-resolution CT. The proposed work will add a fundamental and sensitive new capability to pulmonary disease imaging. Completion of the proposed work will ensure that this method will be ready for translation to a clinical setting, and to the routine application in longitudinal evaluation animal models of pulmonary disease. The proposed work will develop a novel and non-invasive method for imaging pulmonary gas exchange abnormalities. It is particularly relevant for the early diagnosis of interstitial lung diseases such as pulmonary fibrosis. Upon completion of the project, the method should be ready for translation to a clinical setting. [unreadable] [unreadable] [unreadable]